This invention relates generally to conveyors for advancing coated cans through drying or curing chambers, and more particularly to a conveyor system for concurrently rotating the advancing cans in a selected direction and at a controllable rate to ensure proper drying or curing of the cans.
Metal cans for beverages and food products are now mass-produced at exceptionally high rates. Many commercially-produced cans are fabricated by drawing and ironing a metal blank formed of aluminum or steel in a multi-phase operation, thereby avoiding the need for seaming. But before the cans are filled and a lid bonded thereto, it is customary to protectively coat and decorate the outer surface thereof. Such coatings are usually in three layers, the first being a base coat which is a uniform white or ground color. The second layer is the print coat in which printed and decorative matter is laid over the base coat. (In some instances, the print coat is applied directly to the metal can.) Finally, a third layer is applied, whose purpose is to protect the print layer, this overcoat being clear lacquer.
The coatings are generally of the lacquer type which dry or cure by evaporation of the volatile components. The film-forming constituent is usually a high molecular weight polymer, such as a polyester. Other types of lacquer coatings are based on acrylic resins. The solvents used are generally of the low-boiling type, such as aromatics.
It is essential, after each coating is laid down on the can surface, that it be dried to specifications. In some instances, the coatings must be thoroughly dried, in which event even the slightest tackiness is objectionable. In other cases, some residual wetness is acceptable. To this end, it has heretofore been the practice in order to achieve a high-production rate, to convey a continuous train of cans on a conveyor chain through an elongated oven in which the chain travels up and down through a sinuous path to lengthen the exposure time of the cans to the heat without unduly extending the length of the oven.
The heat supplied to such ovens is in indirect form in that air is first heated by a gas-flame burner external to the oven chamber and then blown into the oven at an elevated temperature sufficient to dry the fresh coatings on the cans passing therethrough but well below the ignition point of the coatings.
The existing indirect-heating technique for drying can coatings is inherently inefficient and wasteful in terms of energy expenditure. The heat to volatilize the solvents is derived from the heated atmosphere of the oven, whereas the heat to elevate the temperature of this atmosphere is derived from a gas-flame heater. Only a fraction of the thermal energy supplied by the gas-flame heater is exploited, for most of this energy is dissipated in heating a huge volume of air.
In the copending application of Edward S. Flynn, Ser. No. 630,350, filed Nov. 10, 1975, entitled "Direct Flame Apparatus for Drying Can Coatings," an apparatus is disclosed for drying a wet coating on the surface of cans, the apparatus including a conveyor for advancing the cans in a continuous train through a chamber having a series of direct-flame burner assemblies mounted thereon. Each can, in the course of its advance through the tunnel, is progressively heated by the assemblies to cause the cans to emerge from the chamber in a dried condition.
Inasmuch as the direct flame heater assemblies act to emit thermal energy mainly in one direction, in order to uniformly dry the cans, it is desirable to rotate the cans as they advance through the chamber so that all surfaces of the cans are exposed to the direct flame treatment.
The need to rotate cans as they advance through a treatment chamber also arises in other systems in which coated cans are subjected to other directed forms of energy. Thus in U.S. Pat. No. 3,840,999 of Whelan, there is disclosed apparatus for curing solvent-free inks and other solvent-free coatings, use being made for this purpose of an ultra-violet (UV) radiation-emitting curing lamp which emits UV energy principally in one direction.
In the conveyor disclosed in the Whelan patent, each can advancing through the chamber is supported on a mandrel carried by a motor-driven sprocket chain, the mandrel shaft having a pinion secured thereto which engages a fixed rack. As a consequence, the mandrel is caused to rotate as the can is advanced through the chamber, thereby exposing the entire surface of the can to UV radiation to effect curing of the can coating.
The drawback of this known arrangement is that rotation is necessarily in one direction and the rate of rotation is directly proportional to the speed of advance. Hence it is not possible to change the direction of can rotation or to vary the rate thereof independently of the conveyor speed so as to optimize the curing action.
While the present invention will be described in connection with a direct-flame drying technique as well as with a UV curing arrangement, it is to be understood that it is also applicable to any drying or curing arrangements using infra-red or other forms of directed energy which require can rotation in order to obtain a uniform drying or curing action.
It is also to be understood that the term "coating" as used herein is intended to encompass any lacquer, decorative layer, printing ink or other material applied to the surface of a can and that it includes solvent-free coatings as well as coatings incorporating volatile solvents.